The present invention relates to a method for controlling the gain of a radio-frequency signal, to a transmitting and/or receiving unit and to a communications system.
Particularly in the case of mobile radio links, modulation methods which influence both the amplitude or envelope curve and the phase angle of the signal to be transmitted are used in order to increase the transmission rate. Examples of this are the relatively modern variants of the Global Standard for Mobile Communication (GSM), such as Enhanced Data rate for GSM Evolution (EDGE), and Code-Division Multiplexing (CDM), or Code-Division Multiple Access (CDMA).
A transmitter for a mobile radio link such as this is subject to stringent linearity requirements, in order to prevent errors during transmission. A significant proportion of the nonlinearity of a transmitter such as this is caused in the transmitter amplifier. According to the prior art, the so-called polar loop concept is used, for example, in order to linearize this transmitter amplifier. In this method, the amplitude modulation is applied to the transmission signal separately from the phase modulation, with two separate control loops being provided for this purpose. The method can be implemented in a polar loop transmitter by, for example, modulation of the supply voltage of a power amplifier which is operated highly nonlinearly in the C, D or E mode. On the basis of the polar loop concept, optimized efficiency transmitter amplifiers can be used which, in addition to compliance with a required linearity, also reduce the current that is drawn. Since, particularly in a mobile radio link, at least one subscriber terminal is in the form of a mobile telephone, this considerably reduces the costs for a rechargeable battery for a mobile telephone, and/or increases the maximum operating time of a charged battery set.
If the transmitter amplifier is mismatched as a result of impedance changes in an antenna, such as those which occur in the case of large antennas as a result, for example, of a wind load and in the case of mobile telephones as a result, for example, of a change in the distance between the head and the antenna, the control gradient of the transmitter amplifier changes. The control loop bandwidth of the amplitude control loop is, thus, also changed. This leads to undesirable distortion of the output voltage which is evident, in particular, in a deterioration in the modulation spectrum, which may be sufficiently severe to contravene the spectrum mask which is subject to a fixed specification by virtue of a standard in this field.
One known solution to this problem is to isolate the transmitter amplifier from the antenna through the use of circulators or isolators. However, circulators are relatively narrowband components, so that two or more circulators must be interconnected in order to cover a wider bandwidth. Furthermore, these components are very expensive and, in addition, occupy rather a large amount of space, as a result of which the actually highly advantageous polar loop concept is not financially acceptable, particularly in so-called multiband appliances, owing to the use of a number of circulators.
In the radio transmission systems which have been mentioned above, by way of an example, the modulation of the transmission signal includes amplitude modulation. The modulation may be provided, for example, by variation of the drain voltage in the case of field-effect transistors, or of the collector voltage in the case of bipolar transistors and the anode voltage when electronic valves are used in the transmitter amplifier. Parasitic phase modulation occurs in this case because the phase of the complex transfer function S21 is dependent on the drain/source voltage UDS or UD or on the collector/emitter voltage UCE. If the modulation that is used in the radio transmission system is a combination of amplitude modulation and phase modulation, as is the case, for example, in G2.5 and G3 mobile radio systems, then the phase modulation component is corrupted by an additional parasitic phase modulation component Δφ(A(t)). This corruption results, for example, in an increased error vector and/or an increased bandwidth requirement.
During operation of the transmitter amplifier for a mobile radio link, the respectively involved control loops may be affected both as a result of parasitic phase modulation and by impedance changes in an antenna or the like.
The present invention is, therefore, directed toward a method and an apparatus for improved utilization of a polar loop concept in a transmitter; in particular, for a mobile radio link.